1. Field of the Invention
The invention relates to improved drilling fluids used in the drilling of subterranean oil and gas wells as well as other drilling fluid applications and drilling procedures.
In rotary drilling there are a variety of functions and characteristics that are expected of a drilling fluid ("drilling mud" or simply "mud"). The drilling fluid is expected to carry cuttings from beneath the bit, transport them up the annulus, and permit their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. Likewise the drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated and likewise to form a thin, low permeability filter cake which seals pores and other openings and formations penetrated by the bit. Finally, the drilling fluid is used to collect and interpret information available from drill cuttings, cores and electrical logs.
Drilling fluids are typically classified according to their base material. In water based muds, solid particles are suspended in water or brine. Oil can be emulsified in the water. Nonetheless, the water is the continuous phase. Oil based muds are exactly the opposite. Solid particles are suspended in oil and water or brine is emulsified in the oil and therefore the oil is the continuous phase. The final class of drilling fluids are pneumatic fluids in which drill cuttings are removed by a high velocity stream of air or natural gas. On both offshore and inland drilling barges and rigs, drill cuttings are conveyed up the hole by a drilling fluid.
It is apparent to anyone selecting or using a drilling fluid for oil and gas exploration that an essential component of a selected fluid is that it be properly balanced to achieve the necessary characteristics for the specific end application. As stated hereinabove, the typical compositions include oil based muds, water based muds and pneumatic fluids. For purposes of this application, only oil and water based mud systems will be relevant. The vast majority of oil and gas exploration is done with water based muds. The primary reason for this preference is price and environmental compatibility. Traditional oil based muds made from diesel or mineral oils, while being substantially more expensive than water based drilling fluids, are environmentally incompatible. As a result, the use of oil based muds has been historically limited to those situations where they are necessary.
The preferred embodiment of this invention relates to a novel aqueous drilling fluid including a novel mixture of water soluble monomers after polymerization. A general problem has been recognized in that commercially available additives to drilling fluids have demonstrated unsatisfactory stability when used in fluids that are highly contaminated, for example, with sea water. The consequent instability severely decreases the functional characteristics of the drilling fluid system. Specifically, the viscosity, gel strength, filtrate loss and contamination control characteristics of the drilling fluid are often not maintained within acceptable ranges.
Various water soluble polymers have been synthesized or otherwise developed, and certain polymers are known to occur naturally, some of which have shown at least a limited ability to control the viscosity, gel strength and filtrate loss of aqueous drilling fluids. However the ionic sensitivity and/or temperature stability of these materials is unacceptable, making their use in drilling activities of limited value. During the drilling of certain deep wells, i.e., in excess of fifteen thousand feet, or drilling in geographic areas of high geothermal activity, the viscosity, gel strength and fluid loss of the drilling fluid are adversely affected as a function of the elevated temperature. The noted functional characteristics are consequently outside acceptable ranges.
The drilling fluid itself is an essential item in the oil-well drilling system. In rotary well drilling, the principal functions performed by the drilling fluid are to carry cuttings from beneath the drill bit, transport the cuttings from the annulus and permit their separation at the surface.
The drilling fluid, or drilling mud as it is more typically called, also cools and cleans the drill bit, reduces friction between the drill string and sides of the drill hole and maintains the stability of uncased sections of the borehole. Annular hole cleaning and maintaining wellbore stability are always prominent concerns when developing drilling fluids.
It is essential that the drilling fluid formulation be such that it will prevent the inflow of fluids, such as oil, gas or water, from the permeable rock formations which have been penetrated or which are being penetrated.
The drilling fluid should also contain additives which permit the formation of a thin, low permeability filter cake which seals pores and other openings in the formations which are penetrated by the bit. Finally, the drilling mud must assist in the collection and interpretation of information available from drill cuttings, cores, and electrical logs.
There are certain limitations which are placed on the formulation of muds (drilling fluids) for actual commercial use. The drilling fluids must be formulated such that they are not injurious to the drilling personnel and not damaging or offensive to the environment. The drilling fluids must not cause unusual or expensive methods of completion of the drilled hole nor interfere with normal productivity of the fluid bearing formation. Finally, it is essential that the drilling fluid not corrode or cause excessive wear to drilling equipment. From these requirements has arisen the need for specialized drilling fluid additives as essential components of drilling muds and therefore drilling fluid systems which will assist the drilling mud formulation in the performance of these various functions.
The effectiveness of a drilling fluid and in particular the additives found in the drilling fluid are evaluated by measurement of certain characteristics of the drilling system. The viscosity, gel strength, filtrate loss, contamination control and tolerance to divalent ion characteristics of drilling fluids and drilling systems are all directly attributable to the components of the drilling fluid or drilling mud. These properties, their definitions and a general explanation is found in a comprehensive treatise entitled Composition and Properties of Drilling and Completion Fluids, 5th Ed., George R. Gray and H. D. H. Darley, Gulf Publishing Company, (1988).
One of the essential ingredients of the drilling fluid of this invention is a polymer that enhances the functional capability of the entire drilling fluid system. In formulating a polymer for use as a drilling fluid additive, it is thus necessary to consider the desired functional characteristics of the drilling fluid. The polymers that have shown utility in the drilling fluid of this invention show relatively less sensitivity to salts, divalent cations or solids and therefore better retain their functional properties when compared to current commercially available polymers.
The first essential characteristic of the drilling fluid which is controlled by the polymer additive of this invention is its viscosity. The viscosity of drilling fluids is very difficult to control because of the adverse conditions under which drilling fluids are used as well as the excessively elevated temperatures to which they will be exposed. In this regard, during the drilling of certain deep wells, i.e., greater than 15,000 feet, it is common to be exposed to temperatures at which thermal decomposition of certain drilling fluid additives occurs. These temperatures can easily cause a severe change in the viscosity of the drilling fluid and thus adversely affect the flow characteristics of the drilling mud and adversely affect the overall drilling operation. Such viscosity modification at these temperatures is not acceptable in normal drilling fluids. Additionally, certain areas of the country have excessive geothermal activity resulting in extremely high temperatures. The effect on drilling fluids at these geothermally elevated temperatures may be similar to the effect of elevated temperatures in deep wells.
In any event it is necessary that the viscosity of the drilling fluid be controlled within desired ranges, which are in many instances dependent on the geographic area of activity. The viscosity is a function of plastic viscosity and yield point. As a general rule, as the mud weight increases, the plastic viscosity increases, but the yield point increases by a much smaller magnitude.
A second essential characteristic is the gel strength of the drilling fluid. Gel strength is a characteristic of the drilling fluid which reflects the ability of the drilling fluid to maintain a suspension of additives and drill cuttings, especially when circulation is stopped. As can be appreciated, if circulation of the drilling fluid were terminated, and if all of the suspended cuttings and additives to the drilling fluid were then permitted to settle to the lowest point the drill bit and drill string would be literally packed into a position that would require severe levels of torque to rotate. Such torque might damage components of the drill string or in some instances, cause the drill string to shear apart. Such a situation results in loss of the drill bit and sustained periods where positive footage is not being drilled.
If the drilling fluid gel strength is too low, it is typically increased by adding bentonite. Ideally, the drilling fluid gel strength should be just high enough to suspend barite and drill cuttings when circulation is stopped. Higher drilling fluid gel strengths are undesirable because they retard the separation of cuttings and of entrained gas at the surface, and also because they raise the pressure required to reestablish circulation after changing bits. Furthermore, when pulling pipe, a high gel strength may reduce the pressure of the mud column beneath the bit because of a swabbing action. If the reduction in pressure exceeds the differential pressure between the mud and the formation fluids, the fluids will enter the hole, and possibly cause a blowout. Similarly, when running pipe into the hole, the downward motion of the pipe causes a pressure surge which may, when conditions are critical, cause fracturing with consequent loss of circulation. Methods have been developed for calculation of the magnitude of these pressure surges.
Related to the gel strength control is the ability of the drilling fluid to tolerate divalent ions. Typically, dispersants are used to reduce the gel strength of fresh water or low salinity muds. The use of some types of such dispersants has an unfortunate secondary effect, i.e., the replacement of calcium or other polyvalent cations on clay cuttings by the sodium used to solubilize these particular thinners. This tends to disperse the clay into small particles, some of which are not removed at the surface, and are again recycled until they are reduced to colloidal size. This action makes the control of viscosity very difficult and expensive when drilling through colloidal clay formations with a fresh water mud. The dispersive effect of the sodium ion may be offset by the addition of a calcium compound or else by the use of a polymer brine mud.
During drilling operations, it is always anticipated that the drilling fluid will be contaminated with various materials. Some of the potential contaminants are damaging to the ionic balance and viscosity of the drilling fluid. Such impurities include sodium chloride, gypsum as well as other minerals, magnesium and the like. The drilling fluid additive of this invention displays a high tolerance to divalent cations and ions, especially calcium and magnesium.
Another essential function of the drilling fluid is its ability to seal permeable formations exposed by the bit with a low permeability filter cake. Fluid loss from the borehole is therefore reduced. In order for a filter cake to form it is essential that the drilling fluid contain particles of a size only slightly smaller than that of the pore openings of the formation. These particles are trapped in the surface pores while finer particles are carried deeper into the formation. The particles which are deposited on the formation are known as the filter cake.
It is essential to control the permeability of the filter cake. This permeability depends on the particle size distribution of solids suspended in the drilling fluid and on electrochemical conditions. In general, the more particles there are in the colloidal size range, the lower the cake permeability. The presence of soluble salts in clay bearing muds increases the permeability of the filter cake sharply. Filtration performance in the well is rountinely judged by means of the standard American Petroleum Institute filtration test. In this test, the mud is subjected to static filtration through filter paper for thirty minutes, and the volume of filtrate and the cake thickness are then measured.
Within any specific drilling fluid and drilling system it is necessary to carefully control the filtrate loss within predesigned tolerance ranges. These tolerance ranges will vary from geographic area to geographic area depending on the type of formations encountered.
Oil and water base muds have contrasting attributes and disadvantages. Although drilling fluids utilizing an oil based mud inhibit wellbore swelling by minimizing dispersion fluid, the environmental toxicity of oil muds often overshadow the positive features. Oil base systems can be created with low toxicity but all systems are pollutants to varying degrees. In addition, the cost parameters of an oil mud are often prohibitive when compared to a water based system. Furthermore, the rheological and thixotropic character of an oil mud is not as versatile for maximized hole cleaning as certain aqueous fluids.
Cost effectiveness and environmental acceptability are major advantages of an aqueous drilling fluid but one major disadvantage also exists. Swelling and/or dispersion of formation clays causes general instability of the wellbore with resultant hole enlargement.
Inhibitive materials such as lime, gypsum, tannates and tannate/chrome lignosulfonate systems are used to minimize the factors causing an unstable wellbore.
Due to their chemical nature, these materials require frequent maintenance treatments and/or sufficient sodium hydroxide for solubilization. The resultant hydroxide radical ion causes the dispersion of the clay material. The degree of dispersion is proportional to the hydroxide radical concentration. The dispersion is a major cause of the high solids content of tannate and chrome lignosulfonate systems. A high quantity of low gravity solids content has detrimental effects on another parameter also. There is direct correlation between high quantities of low gravity solids content and reduced rates of penetration.
In an effort to minimize the dispersion and swelling of the formation clays by an aqueous drilling fluid, systems have been developed which rely heavily on polymers and/or soluble salts for inhibition of swelling and/or dispersion of clays. Examples of the polymers used are derivatives such as partially hydrolyzed polyacrylamide ("PHPA") type or polyacrylamide-acrylate copolymers. The aqueous phase can be composed of potassium, sodium or calcium chloride brine, sea water or fresh water.
The mechanism of stabilization achieved by the polymers is not entirely understood. The more successful polymers are anionic polyelectrolytes which are adsorbed at the positive sites of the clay particle. Theoretically, the polymer is adsorbed at multiple points binding the clay material together creating an encapsulation effect.
Alkaline hydrolysis and thermal stability of drilling fluids, and specifically the polymers in drilling fluids, has long been acknowledged as a severe problem in formulating drilling fluid additives. Examples of such drilling fluid additives which are available commercially are PHPA (partially hydrolyzed polyacrylamide) as well as modified starches and cellulose. The partially hydrolyzed (30%) polyacrylamide (PHPA) polymer has become increasingly popular over the last few years. The reasons are varied: (1) promotion of shale inhibition; (2) ease of handling; (3) cost effectiveness; and (4) environmental acceptability. However, the PHPA products are very susceptible to alkaline hydrolysis, particularly at high temperatures.
The polymers that have demonstrated utility in the drilling fluid of this invention are relatively tolerant to such alkaline hydrolysis and hence demonstrate a superior thermal stability during actual use conditions. Polyanionic starches and cellulose products are susceptible to the same thermal stability problems as PHPA. PHPA products are also susceptible to divalent cations and other electrolytes such as sodium chloride.
Although fresh water drilling systems utilizing the PHPA polymer additive offer good shear thinning properties in the higher shear rate range, in the presence of electrolytes and cations, i.e., sea water environment, a significant reduction in the carrying capacity in the low shear rate range occurs. The dynamic carrying capacity which is related to the static suspension qualities results from the interaction of attractive and repulsive surface forces.
The magnesium ions contained in sea water have a particularly detrimental effect on the performance of PHPA in sea water. This often necessitates the cumbersome hauling of fresh water to drilling sites that are contaminated by sea water or chemical treatment of the magnesium ions ("treating out") in the contaminated drilling fluid by chemical treatment. Lime or caustic is generally added to treat out magnesium ions in the drilling fluid. Other limitations of the PHPA drilling fluid additive include sensitivity to low gravity reactive solids and thermal instability above 300.degree. F.
Biopolymers comprise another group of additives including, for example, xantham gum, and guar gum, which also have limiting factors. These deficiencies, the foremost of which are biodegradation and thermal decomposition beginning at a temperature as low as 200.degree. F., result from their composition. The use of such biopolymers in sea water muds may result in intolerably high levels of viscosity.
It is quite obvious that the deficiencies of current drilling fluid systems create a need for a new generation polymer system which addresses many of these problems.
The drilling fluid of this invention, which includes the novel water soluble polymer of this invention provides more effective control of the viscosity, gel strength and fluid loss associated with salt, divalent cation, and electrolyte contaminated systems frequently encountered during well drilling. Likewise, the drilling fluid of this invention operates normally in the neutral pH range of 7-10 and operates favorably in a high alkaline environment, i.e., pH of 10-12 or greater.
The aqueous based drilling fluid and water soluble polymer of this invention effectively control the viscosity, gel strength and fluid loss of an aqueous drilling fluid when exposed to high temperatures.
The aqueous based drilling fluid and water soluble polymer of this invention will, likewise, effectively control the viscosity, gel strength and fluid loss of an aqueous drilling fluid which becomes contaminated with low gravity solids or contains high gravity solids such as barium sulfate. The aqueous based drilling fluid and water soluble polymer of this invention will produce flat instantaneous gel strengths to suspend solids, especially in a deviated or horizontal wellbore thus preventing or minimizing the formation of a cuttings bed. Furthermore, the drilling fluid of this invention will maintain viscosity, or relatively high shear stress values in the low shear rate range thereby increasing the carrying capacity of the drilling fluid under such conditions.
By better controlling the adverse effects of conditions frequently encountered during drilling operations, it is the general objective of the drilling fluid of this invention to obtain a superior degree of borehole stabilization, rheological and supplemental filtration control, and environmental acceptability.
2. The Prior Art
Prior art which is material to the concept of this invention include U.S. Pat. Nos. 3,039,529; 4,076,628; 4,146,690; 4,482,682 and Journal of Applied Polymer Science, Vol. 26, pg. 1117 (1981). Also, U.S. Pat. No. 4,812,544 discloses one or more of the polymers used in the drilling fluid of this invention although there is no effective disclosure of a drilling fluid or incorporation of the polymer into a drilling fluid.